1. Field
In certain embodiments, the invention relates to renewable, cost-effective and low emission fuels, including those used in heating power generation, and transportation.
2. Summary of the Related Art
Combustion of petroleum based fuels (fossil fuels) contribute to increased levels of carbon dioxide (CO2), carbon monoxide (CO), particulate matter (PM), nitrogen oxides (NOx), sulfur oxides (SOx) and other emissions in the earth's environment, which cause respiratory health effects, illnesses and contribute to climate change. Globally, major initiatives are underway to regulate power and transportation emissions using a combination of fuel quality controls, combustion aftertreatment requirements, and consumption mandates. Limiting sulfur content in petroleum fuels has helped to reduce particulate and acid rain causing pollutants, but results in poor fuel lubricity and increased fuel costs, thereby placing a greater operational demand on the end user. Alternative fuels, such as biodiesel, are attractive because they are inherently low in sulfur and reduce PM, CO and hydrocarbon (HC) emissions compared to low-sulfur diesel. These fuels typically contain heteroatoms, such as oxygen, which increases fuel lubricity extending equipment lifetimes.
Biodiesel is typically derived from renewable feedstock; for example, plants, animal fat, microorganisms, or other organisms, that have either fatty acid or lipid (triglyceride) structures. The feedstock are commonly converted into long-chain (fatty) methyl- or ethyl-esters for use in heat, power and transportation applications. Biodiesel can be used as a stand-alone fuel or as a blend with petroleum-based fuels. A common designation for these types of fuel blends is BX, where X is between 0-100 and represents the percent volume of biodiesel in the mixture.
As a comparison, hydrocarbon fuels derived from petroleum are compounds that do not contain oxygen and are non-polar. Biodiesel fuels contain oxygen functional groups, alkyl esters, which are polar. Biodiesel contains only trace amounts of sulfur and aromatic molecules, whereas many petroleum fuels contain significant amounts of sulfur and aromatic molecules. Resulting physical and chemical properties of a hydrocarbon are largely determined by the presence of oxygen, sulfur, aromatic functional groups, and polar functional groups. Therefore, petroleum derived fuels typically are low in specific gravity, are energy dense, have low flash points and exhibit low viscosity. Biodiesel, and its blends with petroleum fuels, exhibit higher specific gravity, lower energy density, higher flash points and increased viscosity. A summary of the differences between biodiesel and petroleum distillate fuel properties can be found in Table I. Because biodiesel and petroleum fuels have different physicochemical properties, fuel handling system materials are often incompatible. For example, accelerated degradation of naturalized rubber hose-liners and gaskets has been documented while utilizing biodiesel.
TABLE IComparison of the physical and chemical propertiesof biodiesel with #2 petroleum distillates demonstratingthe unique characteristics of biodiesel.#2 Pe-troleumDistil-Bio-PropertyTest MethodlatesdieselHeating Value (MJ/kg)ASTM D24046-4840-42Carbon Number (mol-C/mol)N/A 8-2218-25Oxygen:Carbon Ratio (mol/mol)ASTM D52910~0.11Hydrogen:Carbon Ratio (mol/mol)ASTM D5291~1.8~1.9Flash Point (° C.)ASTM D93<73100-170Cetane Number (Rating)ASTM D97540-5548-65Autoignition Temperature (° C.)ASTM E659~315~150Molecular Weight (Ave.) (kg/kMol)N/A~200270-300Kinematic Viscosity @ 40° C. (cSt.)ASTM D4451-33-5Specific Gravity @ 25° C.ASTM D1298~0.84~0.88Aromatics (wt./wt.)ASTM D6591<350
As biodiesel is harvested from living organisms the net carbon foot print is below that of petroleum-based fuels when combusted. Biodiesels, however, offer poor cold weather performance and reduced shelf-life resulting from crystallization and oxidation processes, respectively. As such, biodiesel can benefit from additives or chemical modifications to increase fuel shelf-life, and improve cold weather performance. Oxidation is the primary ageing mechanism of biodiesel, especially the transformation of the unsaturated fatty acid esters inherent in the biodiesel. Saturating the fatty acids by hydrogenation can increase storage stability and reduce oxidation rates; however, this adversely affects resulting cold weather performance as wax-like molecules are prone to crystalize and may block fuel filters. Similarly, metals such as zinc and copper within fuel handling components have been shown to increase the rates of oxidation contributing to fuel aging concerns. Biodiesel additives have been developed to minimize these concerns. Typical storage additives are anti-oxidants, such as tert-butylhydroquinone (THBQ) or butylated hydroxyanisole (BHA), which increase shelf-life. Cold weather additives, such as neopentilglycol and trimethylol propane fatty esters, reduce crystallization temperatures of the fatty esters and can extend the operational temperatures of the fuels.
The combustion of biodiesel (B100) has been studied for its environmental benefits compare to petroleum-based diesels. The U.S. EPA, as defined by the Renewable Fuel Standard, has determined that the net reduction in greenhouse gas emissions from vegetable based biodiesel is approximately 50% on a life-cycle basis. Further, reductions in HC emissions have been reported as high as 67%, accompanied by reductions in CO and PM of approximately 50%. Conversely, NOx emissions are reported to increase by as much as 10%. Since NOx is a known contributor to smog and ground level ozone, methods to reduce the formation of NOx during combustion is desirable. Emulsification of water in biodiesel has been demonstrated to incrementally reduce NOx emissions associated with biodiesel combustion. Water, in amounts up to 20% (vol/vol) of the overall mixture, can either be stabilized in the fuel at the point of storage using surfactants or can be introduced at the point of consumption. Using this method, NOx and PM reductions are reduced to levels below either stand-alone petroleum or biodiesel combustion. Utilization of water, however, reduces the energy density of the fuel mixture since water does not participate in the combustion process. Further, water can accelerate corrosion of low-carbon steels if not used or stored appropriately. Alternatively, NOx- and PM-reducing fuel components are desirable.
Glycerol, also called glycerin or glycerine, has the formal chemical name of 1,2,3-propanetriol and is demonstrated to reduce unwanted emissions during combustion processes. Glycerol is commonly produced from renewable, vegetable-based, feedstock. In biodiesel manufacturing, the glycerol is often considered a low-value co-product that ends up in the aqueous processing streams and is either burned directly for process heat or is upgraded and sold into the commodities markets. In order to make glycerol accessible for most power and transportation applications, it must be introduced as a fuel mixture in the form of an emulsion.
Glycerol emulsions have the ability to improve fuel handling properties compared to petroleum-based fuels, such as bulk fluid viscosity and lubricity. Compared to water emulsion fuels, glycerol emulsion fuels offer high bulk fuel energy density (MJ/kg) while reducing emission of NOx and PM. Further, glycerol is compatible with low-carbon steels making the fuel mixtures less corrosive to storage and fuel handling equipment.
Glycerol, however, is less volatile than both biodiesel and traditional petroleum-derived diesel fuel. This may have a detrimental effect on the combustion quality of some glycerol-containing fuel. Therefore, glycerol-soluble additives for combustion improvement, thinning or viscosity modification may be used to increase volatility and improve combustibility of glycerol in a fuel mixture. Examples include: low molecular weight alcohols, ethers, and other glycerol-soluble compounds that reduce glycerol density and improve volatility. Combustion improvers typically take the form of nitrates, nitriles, ethers, furans, and peroxides. Introduction of these agents reduce the rate of emissions of CO and unburned HC. These compounds typically have a carbon number less than 10 and in some instances have carbon numbers less than 5. A characteristic of these materials is that they typically have flash points below 90° C., and in some instances flash points below 60° C. They also can have boiling points below 120° C., and in some instances boiling points below 90° C.
Glycerol emulsion fuels have been demonstrated utilizing petroleum-based distillates. Cognis Corporation described a fuel mixture comprised of liquid petroleum products in the range of 90% and 99% (vol/vol) and glycerol between 1% and 10% (vol/vol) [U.S. Published Patent Application Publication No. US20080110083, hereby incorporated by reference in its entirety]. Other references have described fuel mixtures containing glycerol for various other applications [ See EP1434834B1, EP1950273A1, US20130133245, each of which is hereby incorporated by reference in its entirety].